In manufacturing poles used by utility companies for electrical transmission or for lighting it often becomes necessary to make tubular poles in more than one section. This can be dictated by the maximum shipping length or a restricted length due to galvanizing limitations or from other field requirements. One common method for joining the poles is a slip joint. A slip joint is a friction fit wherein two sections of poles are slipped together with the female section being above the male section. Both sections have exactly the same taper so that they will slide together a certain distance and then stop and, at least theoretically, be tight and in contact along the entire length of the joint. The amount of overlap is normally one and one half times the diameter at the point of the slip.
The tubular pole sections are constructed of a pair of half shells joined by welds at two longitudinally extending long seams. It is very important that the welds joining the two welds must achieve complete penetration and be inspected to very stringent criteria. This is because as the pole bends the forces tend to exert a hoop stress against the female section which could cause it to split apart. Should this happen the split could continue up the pole causing the slip joint to open up and the structure would fall.
The way to get a good weld with complete penetration on the two longseam welds is to use a heavy steel back-up strip. This allows welding with high heat input assuring that there will be good fusion at the base of the weld. However, this technique has been impractical when a slip joint was involved since the back-up strip would interfere with the male to female fit. Therefore, it has been necessary to make this weld without a full back-up strip or to remove the back-up strip after welding. Both options are unsatisfactory. In the first case, it is very hard to get a good weld without a back-up strip and much repair is required. In the second case, removing the back-up strip is an onerous task which must be done inside a confined area and is detrimental to the health and safety of the worker as well as being very difficult to do without damaging the original weld.